1. Field of the Invention
The invention relates to a magnetic resonance apparatus, including a magnet system for generating a steady magnetic field in a measuring space, which steady magnetic field is oriented mainly parallel to one of the axes of an orthogonal coordinate system, a gradient system with gradient coil systems and control means for supplying the gradient coil systems with excitation currents with a predetermined variation as a function of time, the gradient coil systems including a number of linear gradient coil systems, each of which is arranged to generate a main gradient field which is dependent on the location in the measuring space in such a manner that a magnetic field formed by superposition of one of the main gradient fields on the steady magnetic field can be described, as a function of the coordinates of the coordinate system, as a series containing first-order terms and higher-order terms, the first-order terms having predetermined coefficients equal to zero for two of the coordinates.
2. Description of the Related Art
A magnetic resonance apparatus of this kind is known, for example from WO 96/04565 which corresponds to U.S. Pat. No. 5,572,133. The direction of the steady magnetic field is generally referred to as the z-direction in an orthogonal coordinate system, the other coordinates being referred to as x and y. As a rule a linear gradient coil system is provided for each of the three coordinate directions. These gradient coil systems serve to generate main gradient fields whose component in the z-direction is dependent on the three coordinates as G.sub.x x, G.sub.y y and G.sub.z z, respectively, G.sub.x, G.sub.y and G.sub.z being predetermined constants. The magnetic field obtained in the z-direction by superposition of such a main gradient field on the steady magnetic field is linearly dependent on one of the coordinates. However, it is physically impossible to produce an isolated field gradient in the z-direction. Because the magnetic fields satisfy Maxwell's equations, such a field gradient is always accompanied by another field gradient (concomitant field gradient) which extends perpendicularly to the z-direction. The theoretical background of this phenomenon is described in Magn. Reson. Imaging, 8, 33-37 (1990). Because of these fields the amplitude of the field obtained by superposition of the steady magnetic field and the main gradient field is no longer only linearly dependent on a coordinate, but can be described by a series which also contains higher-order terms in the spatial coordinates.
It has been found that the concomitant field gradients may have disturbing effects if the amplitude of the steady magnetic field has a comparatively low value. Some possible consequences are the selection of warped slices, loss of signal-to-noise ratio (SNR) and image artifacts. Disturbing effects of the concomitant field gradients are sometimes also noticeable during some measurements involving higher steady fields (for example, EPI). It has been proposed to solve this problem for EPI experiments by reversing the polarity of the steady magnetic field; in this respect see SMR, 3rd Annual Meeting, 314 (1995). This method does not offer a solution in respect of selection of warped slices. Moreover, this method can be used only for comparatively low field strengths of the steady magnetic field. Switching over B.sub.0 is actually feasible only in that case.